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    <h1 align="CENTER">An In-Depth Discussion of Virtual Host
    Matching</h1>

    <p>The virtual host code was completely rewritten in
    <strong>Apache 1.3</strong>. This document attempts to explain
    exactly what Apache does when deciding what virtual host to
    serve a hit from. With the help of the new <a
    href="../mod/core.html#namevirtualhost"><samp>NameVirtualHost</samp></a>
    directive virtual host configuration should be a lot easier and
    safer than with versions prior to 1.3.</p>

    <p>If you just want to <cite>make it work</cite> without
    understanding how, here are <a href="examples.html">some
    examples</a>.</p>

    <h3>Config File Parsing</h3>

    <p>There is a <em>main_server</em> which consists of all the
    definitions appearing outside of
    <code>&lt;VirtualHost&gt;</code> sections. There are virtual
    servers, called <em>vhosts</em>, which are defined by <a
    href="../mod/core.html#virtualhost"><samp>&lt;VirtualHost&gt;</samp></a>
    sections.</p>

    <p>The directives <a
    href="../mod/core.html#port"><samp>Port</samp></a>, <a
    href="../mod/core.html#servername"><samp>ServerName</samp></a>,
    <a
    href="../mod/core.html#serverpath"><samp>ServerPath</samp></a>,
    and <a
    href="../mod/core.html#serveralias"><samp>ServerAlias</samp></a>
    can appear anywhere within the definition of a server. However,
    each appearance overrides the previous appearance (within that
    server).</p>

    <p>The default value of the <code>Port</code> field for
    main_server is 80. The main_server has no default
    <code>ServerPath</code>, or <code>ServerAlias</code>. The
    default <code>ServerName</code> is deduced from the servers IP
    address.</p>

    <p>The main_server Port directive has two functions due to
    legacy compatibility with NCSA configuration files. One
    function is to determine the default network port Apache will
    bind to. This default is overridden by the existence of any <a
    href="../mod/core.html#listen"><code>Listen</code></a>
    directives. The second function is to specify the port number
    which is used in absolute URIs during redirects.</p>

    <p>Unlike the main_server, vhost ports <em>do not</em> affect
    what ports Apache listens for connections on.</p>

    <p>Each address appearing in the <code>VirtualHost</code>
    directive can have an optional port. If the port is unspecified
    it defaults to the value of the main_server's most recent
    <code>Port</code> statement. The special port <samp>*</samp>
    indicates a wildcard that matches any port. Collectively the
    entire set of addresses (including multiple <samp>A</samp>
    record results from DNS lookups) are called the vhost's
    <em>address set</em>.</p>

    <p>Unless a <a
    href="../mod/core.html#namevirtualhost">NameVirtualHost</a>
    directive is used for a specific IP address the first vhost
    with that address is treated as an IP-based vhost. In 1.3.13
    and later that includes the IP address <code>*</code>.</p>

    <p>If name-based vhosts should be used a
    <code>NameVirtualHost</code> directive <em>must</em> appear
    with the IP address set to be used for the name-based vhosts.
    In other words, you must specify the IP address that holds the
    hostname aliases (CNAMEs) for your name-based vhosts via a
    <code>NameVirtualHost</code> directive in your configuration
    file.</p>

    <p>Multiple <code>NameVirtualHost</code> directives can be used
    each with a set of <code>VirtualHost</code> directives but only
    one <code>NameVirtualHost</code> directive should be used for
    each specific IP:port pair.</p>

    <p>The ordering of <code>NameVirtualHost</code> and
    <code>VirtualHost</code> directives is not important which
    makes the following two examples identical (only the order of
    the <code>VirtualHost</code> directives for <em>one</em>
    address set is important, see below):</p>
<pre>
                                |
  NameVirtualHost 111.22.33.44  | &lt;VirtualHost 111.22.33.44&gt;
  &lt;VirtualHost 111.22.33.44&gt;    | # server A
  # server A                | &lt;/VirtualHost&gt;
  ...                   | &lt;VirtualHost 111.22.33.55&gt;
  &lt;/VirtualHost&gt;          | # server C
  &lt;VirtualHost 111.22.33.44&gt;    | ...
  # server B                | &lt;/VirtualHost&gt;
  ...                   | &lt;VirtualHost 111.22.33.44&gt;
  &lt;/VirtualHost&gt;          | # server B
                                | ...
  NameVirtualHost 111.22.33.55  | &lt;/VirtualHost&gt;
  &lt;VirtualHost 111.22.33.55&gt;    | &lt;VirtualHost 111.22.33.55&gt;
  # server C                | # server D
  ...                   | ...
  &lt;/VirtualHost&gt;          | &lt;/VirtualHost&gt;
  &lt;VirtualHost 111.22.33.55&gt;    |
  # server D                | NameVirtualHost 111.22.33.44
  ...                   | NameVirtualHost 111.22.33.55
  &lt;/VirtualHost&gt;          |
                                |
</pre>

    <p>(To aid the readability of your configuration you should
    prefer the left variant.)</p>

    <p>After parsing the <code>VirtualHost</code> directive, the
    vhost server is given a default <code>Port</code> equal to the
    port assigned to the first name in its <code>VirtualHost</code>
    directive.</p>

    <p>The complete list of names in the <code>VirtualHost</code>
    directive are treated just like a <code>ServerAlias</code> (but
    are not overridden by any <code>ServerAlias</code> statement)
    if all names resolve to the same address set. Note that
    subsequent <code>Port</code> statements for this vhost will not
    affect the ports assigned in the address set.</p>

    <p>During initialization a list for each IP address is
    generated and inserted into an hash table. If the IP address is
    used in a <code>NameVirtualHost</code> directive the list
    contains all name-based vhosts for the given IP address. If
    there are no vhosts defined for that address the
    <code>NameVirtualHost</code> directive is ignored and an error
    is logged. For an IP-based vhost the list in the hash table is
    empty.</p>

    <p>Due to a fast hashing function the overhead of hashing an IP
    address during a request is minimal and almost not existent.
    Additionally the table is optimized for IP addresses which vary
    in the last octet.</p>

    <p>For every vhost various default values are set. In
    particular:</p>

    <ol>
      <li>If a vhost has no <a
      href="../mod/core.html#serveradmin"><code>ServerAdmin</code></a>,
      <a
      href="../mod/core.html#resourceconfig"><code>ResourceConfig</code></a>,
      <a
      href="../mod/core.html#accessconfig"><code>AccessConfig</code></a>,
      <a href="../mod/core.html#timeout"><code>Timeout</code></a>,
      <a
      href="../mod/core.html#keepalivetimeout"><code>KeepAliveTimeout</code></a>,
      <a
      href="../mod/core.html#keepalive"><code>KeepAlive</code></a>,
      <a
      href="../mod/core.html#maxkeepaliverequests"><code>MaxKeepAliveRequests</code></a>,
      or <a
      href="../mod/core.html#sendbuffersize"><code>SendBufferSize</code></a>
      directive then the respective value is inherited from the
      main_server. (That is, inherited from whatever the final
      setting of that value is in the main_server.)</li>

      <li>The "lookup defaults" that define the default directory
      permissions for a vhost are merged with those of the
      main_server. This includes any per-directory configuration
      information for any module.</li>

      <li>The per-server configs for each module from the
      main_server are merged into the vhost server.</li>
    </ol>
    Essentially, the main_server is treated as "defaults" or a
    "base" on which to build each vhost. But the positioning of
    these main_server definitions in the config file is largely
    irrelevant -- the entire config of the main_server has been
    parsed when this final merging occurs. So even if a main_server
    definition appears after a vhost definition it might affect the
    vhost definition. 

    <p>If the main_server has no <code>ServerName</code> at this
    point, then the hostname of the machine that httpd is running
    on is used instead. We will call the <em>main_server address
    set</em> those IP addresses returned by a DNS lookup on the
    <code>ServerName</code> of the main_server.</p>

    <p>For any undefined <code>ServerName</code> fields, a
    name-based vhost defaults to the address given first in the
    <code>VirtualHost</code> statement defining the vhost.</p>

    <p>Any vhost that includes the magic <samp>_default_</samp>
    wildcard is given the same <code>ServerName</code> as the
    main_server.</p>

    <h3>Virtual Host Matching</h3>

    <p>The server determines which vhost to use for a request as
    follows:</p>

    <h4>Hash table lookup</h4>

    <p>When the connection is first made by a client, the IP
    address to which the client connected is looked up in the
    internal IP hash table.</p>

    <p>If the lookup fails (the IP address wasn't found) the
    request is served from the <samp>_default_</samp> vhost if
    there is such a vhost for the port to which the client sent the
    request. If there is no matching <samp>_default_</samp> vhost
    the request is served from the main_server.</p>

    <p>In Apache 1.3.13 and later, if the IP address is not found
    in the hash table then the match against the port number may
    also result in an entry corresponding to a
    <code>NameVirtualHost *</code>, which is subsequently handled
    like other name-based vhosts.</p>

    <p>If the lookup succeeded (a corresponding list for the IP
    address was found) the next step is to decide if we have to
    deal with an IP-based or a name-base vhost.</p>

    <h4>IP-based vhost</h4>

    <p>If the entry we found has an empty name list then we have
    found an IP-based vhost, no further actions are performed and
    the request is served from that vhost.</p>

    <h4>Name-based vhost</h4>

    <p>If the entry corresponds to a name-based vhost the name list
    contains one or more vhost structures. This list contains the
    vhosts in the same order as the <code>VirtualHost</code>
    directives appear in the config file.</p>

    <p>The first vhost on this list (the first vhost in the config
    file with the specified IP address) has the highest priority
    and catches any request to an unknown server name or a request
    without a <code>Host:</code> header field.</p>

    <p>If the client provided a <code>Host:</code> header field the
    list is searched for a matching vhost and the first hit on a
    <code>ServerName</code> or <code>ServerAlias</code> is taken
    and the request is served from that vhost. A <code>Host:</code>
    header field can contain a port number, but Apache always
    matches against the real port to which the client sent the
    request.</p>

    <p>If the client submitted a HTTP/1.0 request without
    <code>Host:</code> header field we don't know to what server
    the client tried to connect and any existing
    <code>ServerPath</code> is matched against the URI from the
    request. The first matching path on the list is used and the
    request is served from that vhost.</p>

    <p>If no matching vhost could be found the request is served
    from the first vhost with a matching port number that is on the
    list for the IP to which the client connected (as already
    mentioned before).</p>

    <h4>Persistent connections</h4>
    The IP lookup described above is only done <em>once</em> for a
    particular TCP/IP session while the name lookup is done on
    <em>every</em> request during a KeepAlive/persistent
    connection. In other words a client may request pages from
    different name-based vhosts during a single persistent
    connection. 

    <h4>Absolute URI</h4>

    <p>If the URI from the request is an absolute URI, and its
    hostname and port match the main server or one of the
    configured virtual hosts <em>and</em> match the address and
    port to which the client sent the request, then the
    scheme/hostname/port prefix is stripped off and the remaining
    relative URI is served by the corresponding main server or
    virtual host. If it does not match, then the URI remains
    untouched and the request is taken to be a proxy request.</p>

    <h3>Observations</h3>

    <ul>
      <li>A name-based vhost can never interfere with an IP-base
      vhost and vice versa. IP-based vhosts can only be reached
      through an IP address of its own address set and never
      through any other address. The same applies to name-based
      vhosts, they can only be reached through an IP address of the
      corresponding address set which must be defined with a
      <code>NameVirtualHost</code> directive.</li>

      <li><code>ServerAlias</code> and <code>ServerPath</code>
      checks are never performed for an IP-based vhost.</li>

      <li>The order of name-/IP-based, the <samp>_default_</samp>
      vhost and the <code>NameVirtualHost</code> directive within
      the config file is not important. Only the ordering of
      name-based vhosts for a specific address set is significant.
      The one name-based vhosts that comes first in the
      configuration file has the highest priority for its
      corresponding address set.</li>

      <li>For security reasons the port number given in a
      <code>Host:</code> header field is never used during the
      matching process. Apache always uses the real port to which
      the client sent the request.</li>

      <li>If a <code>ServerPath</code> directive exists which is a
      prefix of another <code>ServerPath</code> directive that
      appears later in the configuration file, then the former will
      always be matched and the latter will never be matched. (That
      is assuming that no <code>Host:</code> header field was
      available to disambiguate the two.)</li>

      <li>If two IP-based vhosts have an address in common, the
      vhost appearing first in the config file is always matched.
      Such a thing might happen inadvertently. The server will give
      a warning in the error logfile when it detects this.</li>

      <li>A <code>_default_</code> vhost catches a request only if
      there is no other vhost with a matching IP address
      <em>and</em> a matching port number for the request. The
      request is only caught if the port number to which the client
      sent the request matches the port number of your
      <code>_default_</code> vhost which is your standard
      <code>Port</code> by default. A wildcard port can be
      specified (<em>i.e.</em>, <code>_default_:*</code>) to catch
      requests to any available port. In Apache 1.3.13 and later
      this also applies to <code>NameVirtualHost *</code>
      vhosts.</li>

      <li>The main_server is only used to serve a request if the IP
      address and port number to which the client connected is
      unspecified and does not match any other vhost (including a
      <code>_default_</code> vhost). In other words the main_server
      only catches a request for an unspecified address/port
      combination (unless there is a <code>_default_</code> vhost
      which matches that port).</li>

      <li>A <code>_default_</code> vhost or the main_server is
      <em>never</em> matched for a request with an unknown or
      missing <code>Host:</code> header field if the client
      connected to an address (and port) which is used for
      name-based vhosts, <em>e.g.</em>, in a
      <code>NameVirtualHost</code> directive.</li>

      <li>You should never specify DNS names in
      <code>VirtualHost</code> directives because it will force
      your server to rely on DNS to boot. Furthermore it poses a
      security threat if you do not control the DNS for all the
      domains listed. There's <a href="../dns-caveats.html">more
      information</a> available on this and the next two
      topics.</li>

      <li><code>ServerName</code> should always be set for each
      vhost. Otherwise A DNS lookup is required for each
      vhost.</li>
    </ul>

    <h3>Tips</h3>

    <p>In addition to the tips on the <a
    href="../dns-caveats.html#tips">DNS Issues</a> page, here are
    some further tips:</p>

    <ul>
      <li>Place all main_server definitions before any
      <code>VirtualHost</code> definitions. (This is to aid the
      readability of the configuration -- the post-config merging
      process makes it non-obvious that definitions mixed in around
      virtual hosts might affect all virtual hosts.)</li>

      <li>Group corresponding <code>NameVirtualHost</code> and
      <code>VirtualHost</code> definitions in your configuration to
      ensure better readability.</li>

      <li>Avoid <code>ServerPaths</code> which are prefixes of
      other <code>ServerPaths</code>. If you cannot avoid this then
      you have to ensure that the longer (more specific) prefix
      vhost appears earlier in the configuration file than the
      shorter (less specific) prefix (<em>i.e.</em>, "ServerPath
      /abc" should appear after "ServerPath /abc/def").</li>
    </ul>
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